Theory and Method
Two different mechanisms govern how water crosses a geosynthetic barrier, and the GHPT I addresses both. An intact membrane passes water only through permeation, where molecules dissolve into the polymer on the pressurized face, diffuse across the thickness, and leave on the far side. Flaws behave nothing like that. Pinholes, thin spots, and gaps in a laminate bond act as open channels, and water pushes through them in bulk once pressure exceeds what surface tension and geometry can hold back.
Hydrostatic resistance testing looks for that second mechanism. The pump raises pressure in steps, holds each step for a set interval, and the operator watches the downstream collector. Nothing appears while the applied head stays below what the largest flaw resists. Once it crosses that point, water breaks through and the collected volume climbs sharply, marking the material's hydrostatic resistance. Permeability testing works below that threshold and measures steady flow instead, applying Darcy's law as k = QT / (AtΔH), where Q is permeated volume, T thickness, A exposed area, t elapsed time, and ΔH the head difference.
Temperature enters the calculation through viscosity. Warmer water flows more readily through the same channel, so an uncontrolled bench can produce different coefficients for identical specimens tested months apart. The heating and cooling system holds the water at a declared temperature throughout, which makes results comparable and lets laboratories test at the temperature a barrier will actually see in service. Support plate perforations of ø3 mm drain permeated water toward the ø200 mm collector while backing the specimen against 2.5 MPa, so the material never stretches unsupported and fails mechanically instead of hydraulically.
